A vehicle equipped with an ABS control system has been generally designed to be controlled on the assumption that the front wheel is locked prior to the rear wheel. Japanese Patent Application Laid-Open Hei 7-159308 discloses that the wheel speed difference between the front and rear wheels is calculated, and a friction coefficient μ on the road surface is estimated based on a time gradient obtained when the change ratio of the wheel speed difference becomes equal to or larger than a predetermined value. The ABS control operation is executed based on the resultant friction coefficient μ.
In the case where the friction coefficient μ of the road surface is small, each wheel speed of the four wheels may be reduced simultaneously contrary to the assumption that the front wheel is locked prior to the rear wheel. The ABS control serves to estimate the vehicle speed based on the higher value of the wheel speed selected among those of the four wheels. The current slip ratio is derived from the estimated vehicle speed so as to be controlled into the required slip ratio. In the case where the wheel is brought into the slip state without generating the wheel speed difference between the front and rear wheels as described above, the vehicle speed cannot be estimated, thus failing to determine the slip state of the vehicle.
In the case where the four-wheel-drive vehicle is traveling on the road with a considerably low friction coefficient μ, the small reaction force on the road surface is overcome by the force that functions in restricting the four wheels or front and rear wheels. The wheel speeds of those four wheels, thus, are reduced simultaneously.
In the case where the two-wheel-drive vehicle is traveling on the road with a considerably low friction coefficient μ, the force that functions in restricting the front and rear wheels is small. Then the front wheels become likely to be locked, and the rear wheels become likely to be locked thereafter. The rear wheel speed tends to be reduced at an earlier stage far deviated from the vehicle speed. The vehicle speed is estimated on the assumption as being equal to the highest wheel speed selected among those of the four wheels, that is, the rear wheel speed in spite of the low deceleration in the actual vehicle speed. It is not practical to execute the ABS control using the slip ratio derived from the estimated vehicle speed far deviated from the actual value.
Meanwhile Japanese Patent Application Laid-Open Hei 10-6964 discloses a system which determines that all the wheels of the four-wheel-drive vehicle are locked if the estimated vehicle deceleration exceeds a predetermined value and the difference in the rotating speeds between the front and rear axles is within a predetermined range.
The system disclosed in the aforementioned publication is capable of determining the lock state of all the four wheels of the vehicle traveling on the road with the specific friction coefficient μ. The system, however, is not capable of determining the slip state of all the four wheels prior to the determination of the lock state of those wheels.